The output power of a transmitter in a wireless communication link is largely determined by the power amplifier (PA) and the load impedance seen at the output of the PA. For open loop PAs, variations in load impedance will result in variations in the output power. Ideally, the load impedance is constant. However, in reality, the load impedance may vary due to variations in the impedance of the components in the transmit chain following the power amplifier, such as a duplexer, caused by frequency and temperature variations. The load impedance may also vary due to variations in the impedance of the antenna caused by proximity of the antenna to foreign metallic objects and the user's body. For a mobile handset communication link, output power variations decrease the quality of service, increase the probability of dropped calls in fringe areas, increase the liability of the mobile terminal manufactures in terms of tolerated output power versus Specific Absorption Ratio (SAR), and increase the peak current of the PA.
There are several methods to reduce output power variation due to load impedance variation. One method is to insert an isolator between the PA output and the antenna. However, the isolator is an additional component that increases board area and cost. In addition, the isolator adds loss to the transmit path, thereby decreasing the efficiency of the transmitter. Another method is to add a radio frequency (RF) power control loop. Common RF power control loops consist of a directional coupler and a RF power detector, where the output of the RF power detector is used to control the output power of the PA. Like the isolator, a directional coupler is an additional component that increases cost and adds loss to the transmit path, which decreases the efficiency of the transmitter. By using a directional coupler, which is used to separate forward power from reverse power at the PA output, and a power control loop, the transmitter can provide constant forward power to the antenna. However, in order to provide constant delivered power to the antenna, a dual directional coupler is necessary in order to measure the forward and reflected power at the output of the PA. The insertion loss of a dual directional coupler is twice that of a single directional coupler (in dB). As such, a dual directional coupler further decreases the efficiency of the transmitter.
In addition, testing of mobile devices is becoming increasingly more stringent and, as a result, over-the-air (OTA) testing is now being standardized to further test performance. As OTA testing is developing, Total Radiated Power (TRP) is becoming increasingly important. The TRP of a mobile device is an integral measure of the radiation pattern emitted by the device during transmit operation while in a real-world, or a simulated real-world, environment.
Thus, there is a need for a system and method for detecting forward and reverse power of a RF transmitter that is insensitive to variations in load impedance, adds minimal insertion loss to the transmit path, and adds minimal cost.